Account the 7-cycle hardware interrupt entry cost in cycle accounting

[highbyte]

Hardware IRQ/NMI entry previously consumed 0 cycles in cycle accounting; real 6502-family entry takes 7 (two dummy opcode reads, three stack pushes, two vector reads — the bus sequence itself was implemented in #289). Follow-up scheduled from that work.

What changes

• CPU.ProcessPendingInterrupts (and the internal boundary servicing) returns the cycles consumed: new CPU.InterruptEntryCycles (7) when an interrupt was serviced, else 0.
• The entry cost is folded into the preceding instruction's InstructionExecResult (new WithAdditionalCycles) before statistics update, so every cycle-paced consumer sees real elapsed time with no further changes: ExecState totals (Apple II disk motor/speaker/game-port clocks), LastInstructionExecResult (Apple II / VIC-20 frame budgets and VIA ticking), evaluators, and the InstructionExecuted event.
• The C64's post-device-tick interrupt flush explicitly ticks CIA1/CIA2 and the cartridge slot with the entry cycles and includes them in the raster advance and frame budget. The freeze-button NMI delivery deliberately ignores them (one-shot event outside frame pacing; commented).
• Note: on the full Execute path, NmiAcknowledging now fires before InstructionExecuted (servicing moved before stats update); the only NmiAcknowledging consumer (C64) uses the minimal path and is unaffected.

Impact

Frame budgets now fit ~7 fewer CPU cycles per serviced interrupt (~0.04% per raster-IRQ frame on C64) — matching real hardware. All existing C64/VIC-20/Apple II timing tests pass unchanged; benchmarks at parity (hot path gained only a return-value check).

Tests

+4 in CPUInterruptBoundaryTests: IRQ entry costs 2+7 on the minimal path (result and ExecState views), NMI costs 2+7 on the full path, ProcessPendingInterrupts returns 7/0, and a no-interrupt control keeps plain instruction cycles.

Public API note: ProcessPendingInterrupts changed void → ulong (source-compatible for callers ignoring the result; package is alpha).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.0% Coverage on New Code
0.0% Duplication on New Code

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